High speed impact line printers comprise an endless type band with a row of characters moving parallel with a row of print hammers. The band is trained on spaced drive and idler or tension pulleys which are motor driven to revolve the band at constant speed. The hammers and type band are separated by a narrow gap or passageway through which a continuous web which is moved intermittently and an ink ribbon which is constantly in motion relative to the paper. Electronic controls operate the print hammers individually in synchronism with the moving type band to impact the paper and ink ribbon against each other and selected characters on the moving band. It is common practice to provide a separator device, also called a ribbon shield, which operates to maintain the paper and ink ribbon separated except when the hammer impacts the paper against the ribbon and characters. The purpose of the shield is to reduce smudge and ink transfer between ribbon and paper primarily during non-printing intervals. Smudge is usually worse on multi-part forms. It is desirable to have a thick shield to obtain maximum separation to reduce smudge. However, thick shields, in addition to absorbing greater energy from the print hammers, cause underscore on multi-part forms. This underscore is due to the pressure and force of the ribbon shield during impact by the hammers.
Such ribbon shield devices can take various forms. In the case of U.S. Pat. No. 4,437,401, the ribbon shield comprises a separator plate made of plastic foil which is configured in the form of a flattened Z, one section of which is attached to the printer frame and the other section of which presses the paper against the print hammer plate at some position below but near and coextensive with the print line. Another form of ribbon shield comprises a frame member and a elastic shield element attached thereto. The shield element may have a window through which the paper is driven by the hammer against the ribbon and type. Unless the ribbon is maintained taut, a problem with the window shield is snagging of the loose ribbon on the edges of the window. In another form of shield, the shield element is a flat plastic separator sheet attached to a U-shaped frame. The edge of the separator sheet extends across the width of the paper. As in the case of the flat Z separator plate, the plastic sheet needs to be relatively thick to provide adequate separation force but presents the problem of underscoring. The problem of underscoring can be reduced by lowering the edge further below the print line but this reduces the separation in the print zone during non-printing which increases the chances for smudging.